This invention relates to retinal display devices, and more particularly to optical configurations for retinal display devices and a method and apparatus for defining an exit pupil through which a user views an image.
A retinal display device is an optical device for generating an image upon the retina of an eye. Conventional retinal scanning displays use a coherent light source which is scanned in raster fashion onto the retina. Light is emitted from a light source, collimated through a lens, then passed through a scanning device. The scanning device defines a scanning pattern for the light. Following the scanning device, the scanned light passes through an objective lens which converges the light to focus an image. Conventionally such light is converged to a flat image plane. The light then diverges beyond the plane. An eyepiece is positioned along the light path beyond the objective lens at some desired focal length. An xe2x80x9cexit pupilxe2x80x9d (i.e., area of generated light pattern) occurs shortly beyond the eyepiece in an area where a viewer""s eye pupil is to be positioned.
A viewer looks into the eye piece to view an image. The eye piece receives light that is being deflected along a raster pattern. Light thus impinges on the viewer""s eye pupil at differing angles at different times during the scanning cycle. This range of angles determines the size of the image perceived by the viewer. Modulation of the light during the scanning cycle determines the content of the image.
Typically the exit pupil defined by the display device is less than 2 mm in diameter and often less than 1 mm in diameter. The viewer""s eye pupil varies from approximately 2 mm in diameter under bright light to approximately 7 mm in a dark room. Because of the small exit pupil, a first step for a viewer is to adjust eye position to find the exit pupil. The viewer""s pupil needs to achieve and maintain alignment with the display device""s exit pupil. While in alignment, the light scans directly onto the viewer""s retina without any intermediary screens, cathode ray tubes (CRT""s) or liquid crystal display devices (LCD""s). The result is an image perceived by the viewer.
A shortcoming of the conventional retinal display is the difficulty of maintaining alignment between the exit pupil and the viewer""s pupil. If the viewer moves, alignment may be lost. Movement is problematic because a viewer has a tendency to move their eye when intending to view a peripheral portion of the image. Even blinking may cause movement of the eye. As a result, conventional exit pupils are inconvenient for the viewer. In particular a lay consumer using a virtual retinal display would find the alignment requirement difficult to maintain for entertainment or other long term viewing applications. Accordingly there is a need for a retinal display device having an exit pupil defined so as to enable easier viewing of the image.
Other shortcomings of conventional retinal display devices include the display""s size and weight. As the retinal display device is to be positioned in the vicinity of a viewer""s eye, there is a need to achieve a lightweight compact display device.
According to the invention, a lightweight, compact retinal display device is achieved using a simplified optical system which generates an expanded exit pupil without compromising magnification or resolution.
According to one aspect of the invention, a scanning device for deflecting light is located along the light path following an objective lens system. Significantly, the retinal display device of this invention avoids use of an objective lens system following the scanning device. The elimination of an objective lens system beyond the scanner shortens the light path through the retinal display device. One advantage of such a configuration is a lighter, more compact display device.
In a post-objective scanning system the scanning device receives converging light. Beyond the scanning device, the light continues to converge to an intermediate image plane. According to another aspect of this invention, the image plane is an intermediate curved image plane. The light then diverges beyond this plane in the direction of an eyepiece.
According to another aspect of the invention, an apparatus for expanding the exit pupil is positioned between the scanning device and the eyepiece at the curved image plane. To achieve a focused image with maximum resolution, the exit pupil expanding apparatus defines a curved surface which coincides with the intermediate curved image plane. The apparatus is positioned at the intermediate curved image plane so as to maintain maximum resolution and focus.